Generally, a simple single or multi-pass tubular heat exchanger typically consists of a shell (a large vessel) with a bundle of tubes inside of the shell. Two fluids, of different starting temperatures, flow through the heat exchanger. One fluid flows inside of the tubes (typically a gas such as steam) while a second fluid flows outside of the tubes through the shell (typically a liquid such as water. Thus, heat is transferred between the two fluids without direct contact between the two fluids. The fluid flowing inside of the tubes is known as tube-side fluid, while the fluid flowing outside of the tubes is known as shell-side fluid. During normal operation, heat will be transferred from the hotter fluid, through the walls of the tubes, and into the cooler fluid. Depending upon the desired results, heat is transferred either from tube-side to shell-side or vice versa.
Referring to FIG. 1, an example of a typical prior art single-pass heat exchanger 90 is schematically illustrated. At each end of the prior art heat exchanger 90 there is a tube-side fluid plenum 91. In a multi-pass heat exchanger, the plenum is at only one end of the heat exchanger. The plenum 91 is filled with tube-side fluid and a tubesheet 92 is positioned between the plenum 91 and the shell-side chamber 93. The tubesheet 92 generally has a solid outer rim 94 and a perforated zone 95 through which the ends of each tube 96 are connected to the plenums 91. The solid outer rim 94 is connected directly to the shell 97.
One of the challenges to the long-term reliability of such classical heat exchangers comes from the large number of thermal transients. These transients produce severe stresses in the perforated region 95 of the tubesheet 92. In such classical heat exchanger designs, the flow of tubeside fluid through the tubesheet 92, coupled with the reduced metal mass of the perforated zone 95, has the net effect of producing a temperature profile in the interior that is substantially different from the solid rim region 94. Variation in the temperature of both tube-side and shell-side fluids affects the stress field in the tubesheet 92, although to different levels of severity.
Variation in the temperature of the shell-side fluid with time actuates changes in the metal temperature of the tubesheet 92. However, the perforated interior 95 follows the shell-side fluid temperature variation much more closely than the outer rim 94 due to the reduced thermal mass of the former. Different temperature change rates in the rim 94 and in the interior 95 of the tubesheet 92 produce thermal stress variations. The effect of pulsations in the tube-side fluid temperature is usually far more severe. The perforated interior 95 follows the temperature of the tube-side fluid even more closely due to the extensive surface contact between the tube-side fluid and the tubesheet 92 (over the lateral surface, and inside surface of perforations). Thus the temperature ramps of the perforated region 95 and the untubed region can be significantly different. The resulting pulsation in the stresses can cause fatigue failure of the metal in the perforated zone 95, or in the rim 94, depending on the geometric dimensions of the tubesheet 92. If the tubesheet 92 is integrally welded to the channel and (or) the shell 97, then these junctions emerge as the most vulnerable spots.